Next month, a scientific committee sponsored by the United Nations will put out its latest assessment of climate change. The report is expected to underscore yet again that climate change is a serious problem and human beings are largely responsible.

The Intergovernmental Panel on Climate Change (IPCC) represents a consensus view of hundreds of scientists from around the world. The effort shared the 2009 Nobel Peace Prize with Al Gore.

In some ways Kevin Trenberth personifies that consensus. He's been part of the IPCC since its early days in the 1990s and is outspoken in defense of the science.

And it turns out that his own research bears directly on one of today's major talking points by climate skeptics. His studies, at the National Center for Atmospheric Research in Boulder, Colo., seek to explain why the global air temperature hasn't increased over the past 15 years.

Trenberth is the most prominent denizen of the research center — its distinguished senior scientist. You'd never guess he's 68 years old, either by his spry appearance or his scientific productivity.

And while these days he's a staunch advocate for the scientific consensus, his first foray into climate science was a cut across the grain. There was a devastating drought in 1988, which attracted the attention of one of the most renowned climate scientists.

"Jim Hansen famously went before Congress and declared that the drought was due to global warming, essentially," Trenberth says. "And I wrote a paper along with two others that appeared in Science magazine which basically said that it wasn't."

Instead, he says the drought had to do with what at the time was an unappreciated part of the climate system: the El Nino warming phenomenon in the Pacific Ocean.

Understanding Global Temperature Changes

Fast forward 25 years, and Trenberth still sees changes in ocean temperature as key to understanding the ups and downs of global climate. That includes the current plateau in global temperature.

Trenberth says, in fact, the planet has continued to warm during this time — but the heat has been flowing into the oceans, which have a vast capacity to absorb it.

So will the oceans come to our rescue?

"That's a good question, and the answer is maybe partly yes, but maybe partly no," he says.

The oceans can at times soak up a lot of heat. Some goes into the deep oceans where it can stay for centuries. But heat absorbed closer to the surface can easily flow back into the air. That happened in 1998, which made it one of the hottest years on record.

Trenberth says since then, the ocean has mostly been back in one of its soaking-up modes.

"They probably can't go on much for much longer than maybe 20 years, and what happens at the end of these hiatus periods, is suddenly there's a big jump [in temperature] up to a whole new level and you never go back to that previous level again," he says.

You can think of it like a staircase. Temperature is flat when a natural cool spell cancels out the gradual temperature increase caused by human activity. But when there's a natural warm spell on top of the long-term warming trend, the story is dramatically different.

"When the natural variability or when the weather is going in the same direction as global warming, suddenly we're breaking records, we're going outside of the bounds of previous experience, and that is when the real damage occurs," Trenberth says.

Consider Hurricane Sandy. Trenberth figures the storm was maybe 5 or 10 percent more powerful as a result of global warming. And sea level is 8 inches higher than it was a century ago. That doesn't seem that dramatic, but he argues that made a huge and costly difference.

"I reckon that without climate change, we would not have exceeded thresholds that caused the flooding of the subways in Manhattan and the tunnels from Manhattan to New Jersey and to Brooklyn."

It has taken quite a few years for Trenberth and his colleagues to piece together the role of oceans in climate variability. It involved a huge amount of data, taken from ocean buoys that take the temperature of the deep sea, along with satellites that measure energy flowing into and out of the atmosphere.

And a few years ago, Trenberth was lamenting to his colleagues in an email that the Earth observing system still didn't give them all the data they needed to fully explain the ups and downs of global temperatures.

"I said it was a travesty that we couldn't account for, essentially, the global warming in some sense," Trenberth says.

This email ended up being taken from a British computer and published along with a flood of other private conversations, in an episode dubbed "Climategate." His comment was singled out by skeptics, who claimed scientists were covering up the truth about global warming.

"That email was taken completely out of context and misused in many respects," Trenberth says.

An Expanding Role For Scientists

Trenberth readily acknowledges that there are still some gaps in understanding the Earth's overall heat balance. But that doesn't undercut the basic observation that carbon dioxide and other gases from human activity are driving up the Earth's temperature in the long run.

Indeed, the last decade was the warmest on record, even though temperatures didn't keep climbing during that period. The 1990s were warmer than the 1980s. And the 1980s were warmer than the 1970s.

Over the decades that he has been working on climate change, the role of scientists has gradually expanded. Prominent scientists like him are trying to reduce the risk of global disruption by pushing society to act. These are frustrating times.

"This is very much in the role of the politicians who are supposed to do what's in the interests of everybody as a whole," Trenberth says. "And I'm not so sure many politicians understand their role in this."

There's a deep current on Capitol Hill that says it's pointless even to try; because China and India seem destined to produce so much carbon dioxide, curtailing U.S. emissions won't do much at all. But wading into this policy debate, Trenberth argues that the United States could and should lead the world toward a less dangerous trend.

"If you play the right kind of role, then other countries will follow," he says.

You could argue that's simply wishful thinking, or you could argue that China and India would be even less likely to address climate change if the United States wasn't even going to try.

Trenberth has no illusions that we can do anything to stop the climate from changing altogether. After all, nature is changing the climate all the time. But the question now is moderating the speed of that change.

"Some of the human-induced changes are occurring 100 times faster than they occur in nature," Trenberth says. "And this is one of the things that worries me more than climate change itself. It's actually the rate of change that's most worrying."

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